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rfftopt (3)
  • >> rfftopt (3) ( Solaris man: Библиотечные вызовы )
  • 
    NAME
         rfftopt - compute the length of the closest fast FFT
    
         INTEGER FUNCTION RFFTOPT(N)
    
         INTEGER N
    
    
    
         #include <sunperf.h>
    
         int rfftopt(int len);
    
    PURPOSE
         RFFTOPT computes the length of the closest fast  FFT.   Fast
         Fourier  transform  algorithms, including those used in Per-
         formance Library, work best with  vector  lengths  that  are
         products  of  small  primes.   For example, an FFT of length
         32=2**5 will run faster than  an  FFT  of  prime  length  31
         because  32  is a product of small primes and 31 is not.  If
         your application is such that you can taper or zero pad your
         vector  to  a  larger length then this function may help you
         select a better length and run your FFT faster.
    
         RFFTOPT will return an integer no  smaller  than  the  input
         argument N that is the closest number that is the product of
         small primes.  RFFTOPT will return 16 for an input  of  N=16
         and return 18=2*3*3 for an input of N=17.
    
         Note that the length computed here is not guaranteed  to  be
         optimal,  only  to  be a product of small primes.  Also, the
         value returned may change  as  the  underlying  FFTs  become
         capable  of handling larger primes.  For example, passing in
         N=49 today will return 50=2*5*5 rather than  49=7*7  because
         the  FFTs  in  Performance  Library do not have fast radix 7
         code.  In the future, radix 7 code may  be  added  and  then
         N=49 will return 49.
    
    ARGUMENTS
         N         (input) INTEGER
                   Length of the vector to be transformed.  N  >=  0.
                   RFFTOPT(0) .EQ. 0.
    
    
    
    


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